1. Field of the Invention
The present invention generally relates to a method for automatically calibrating an output power of an optical pick-up head, and more particularly, to a method for automatically calibrating an output power of optical pick-up head according to a relationship between the output power for writing an optical disc and a specific parameter.
2. Description of Related Art
Multi-pulse or multi-level write strategies are currently used for optical disc drivers to perform writing operations on optical discs. These two write strategies are featured by controlling analog channels of a laser diode driver from the optical pick-up head, so that the optical pick-up head emits an output power of multi-pulse or multi-level for writing the optical discs.
FIG. 1 is a schematic diagram illustrating a conventional optical pick-up head. Referring to FIG. 1, the optical pick-up head includes digital-to-analog converters 101a, 101b, 101n, a laser diode driver 102, a laser diode 103, a beam splitter 104, a concentrating lens 105, a pre-diode 106, and a one-eighth diode 108. The laser diode driver 102, has a plurality of analog input channels, each of which correspond to an input value, e.g., X1, and an enabling signal, e.g., EN1, of a digital-to-analog converter, e.g., 101a, for controlling a value and time of the output power of the laser diode 103 respectively.
There are different mapping relationships of the input values of the digital-to-analog converters in correspondence with the output power of the laser diode 103. The laser diode driver 102 is adapted to sum up the enabled analog input channels, so as to drive the laser diode 103 to emit laser. The laser is split by the beam splitter 104 into two parts. A first part illuminates on the pre-diode 106, e.g., a photo sensor, a second part passes through the concentrating lens 105 and incidents on the optical disc 107. The second part is then reflected by the optical disc 107 back to the one-eighth diode 108 for further signal processing by the optical disc driver.
However, in production of optical pick-up heads, errors of the optical elements used thereby and tolerances in assembling unavoidably cause deviations between the input values of the digital-to-analog converters and the actual output powers of the laser diode. A typical solution addressing to this issue is to employ an auto power control loop for feeding back an output parameter detected by the pre-diode 106, to compensate the deviation.
FIG. 2 is a schematic structural diagram of auto power control loop. Referring to FIG. 2, the auto power control loop 200 includes a comparator 204, an integrator 205, a sampling maintenance circuit 207, and an amplifier 208. As discussed above, when the laser diode driver 202 drives the laser diode to emit laser, the pre-diode 206 receives a part of the laser emitted from the laser diode.
An operation of the auto power control loop 200 includes the following process steps. First, the pre-diode 206 detects an output parameter FPDO. Next, the output parameters FPDO are sampled by the sampling maintenance circuit 207 and amplified by the amplifier 208 in sequence, so as to obtain an output signal FPDO′. Next, the comparator 204 compares the output signal FPDO′ with an input value X that has been converted by the ADC 201, and then the integrator 205 provides the adjusted input value to the analog input channel of the laser diode driver 202 for automatically calibrating the output power of the optical pick-up head.
Unfortunately, different optical disc drivers are often distinct as having differences in resistance of PCB, ambient noise, and laser diode driving chips. Therefore, even when identical input values of a digital-to-analog converter are provided for controlling the writing of optical pick-up head, the input values often deviate from the expectation when arriving at the digital-to-analog converter, which causes differences between output powers of the laser diode 103. The foregoing auto power control loop is incapable of compensating these differences. Therefore, it is a concern for advancement to study the conversion characteristics of different analog input channels of the optical pick-up head relating to different optical disc drivers, and adaptively adjust the output power of the optical pick-up head, so as to optimize the writing performance of the optical disc driver.
FIG. 3 is a schematic diagram illustrating measuring characteristics of the optical pick-up head using a power meter. Referring to FIG. 3, the optical pick-up head of the optical disc driver 303 includes a pre-diode, for automatically calibrating a magnitude of the laser emitted by the optical pick-up head. A computer 302 provides an input value of a digital-to-analog converter (hereinafter power instruction value) to the optical disc driver 303 via an integrated drive electronics (IDE). Next, a power meter 304 is employed as measuring a power of the laser emitted from the optical pick-up head. An output signal S outputted from the power meter is an analog signal. The output signal S is then converted by a digital-to-analog converter 301 into a digital signal. The digital signal is then transmitted to the computer 302. A relationship between the power instruction values and the output power of the optical pick-up head can be obtained by repeating the foregoing process. The relationship is also known as conversion characteristics. However, the power meter 304 is an excessive device to the optical disc driver. Therefore it is inconvenient for the manufacturers to sequentially test the conversion characteristics with such a power meter.
FIG. 4 is a schematic diagram illustrating measuring characteristics of the optical pick-up head using a bar code. Referring to FIG. 4, a bar code is stuck to an optical pick-up head of the optical disc driver 403 by the manufacturer of the optical disc driver 403. The bar code contains information about a relationship between an output current of the pre-diode and the output power of the optical pick-up head, which is also named as sensitivity thereof. For example, the information contained in the bar code is a ratio between the output current of the pre-diode and the output power of the optical pick-up head. In such a way, a bar code reader can be used for retrieving the information from the bar code and thus obtaining the sensitivity related information and transfer the same to the computer 402. When the computer 402 provides a power instruction value to the optical disc driver 403 for writing, the relationship between the power instruction value and the output power of the optical pick-up head can be obtained referring to the sensitivity and the output current of the pre-diode.
However, the bar code reader is also an excessive device. In related art, U.S. Pat. No. 6,621,778 proposes an automatic calibration of the output power of a pickup head for an optical information recording apparatus. It assumes that characteristics of pre-diodes are similar, and according to which the method selects a plurality of pre-diodes and measures relationships between output currents of the pre-diodes and the output power of the optical pick-up head, and then calculate an average thereof. Each optical disc driver obtains the relationship between the power instruction value and output power of the optical pick-up head, according to the average and the output current of the pre-diode, when receiving power instruction value for writing. However, such a method relying on the average for calibrating output power of the optical pick-up head is less precise.
Further, U.S. Pat. No. 6,944,109 proposes an automatic control method for generating stable laser power of an optical disc driver, in which a plurality of optical disc drivers are tested as samples for reference. Relationships between output parameters of the pre-diodes and the power instruction values are obtained and averaged, so as to obtain a standardized curve. When the optical disc driver receives an power instruction value for writing operation, the relationship between the output parameter of the pre-diode and the power instruction value is measured and compared with the standardized curve, and thus obtaining a deviation function thereby. The deviation function is adapted for compensating the power instruction value so as to control or calibrate the output power of the optical pick-up head.